We use stable carbon and nitrogen isotopes to investigate how forest fragmentation affects niche partitioning among sympatric dwarf and mouse lemur species inhabiting three eastern Malagasy rainforests with varying levels of disturbance (Tsinjoarivo continuous forest, Tsinjoarivo fragmented forest, and Ranomafana selectively-logged forest). Overall, carbon isotope values in lemurs roughly correlate with those in plants, and nitrogen isotope values are similar for plants and lemurs at Ranomafana and Tsinjoarivo continuous forest. These results suggest that cheirogaleid foraging is not affected by the historical logging activity at Ranomafana. However, nitrogen isotope values for lemurs and plants diverge at Tsinjoarivo fragmented forest. Whereas plant nitrogen isotope values are lowest at this locality, those for lemurs are elevated. This pattern strongly suggests that lemurs behave differently in the fragmented forest. Carbon data indicate that mouse lemurs may feed at lower canopy heights than dwarf lemurs at all three localities. They also suggest that sympatric Cheirogaleus crossleyi and C. sibreei might feed at different canopy heights in the forest fragment. Mouse lemurs have significantly higher nitrogen isotope values than dwarf lemurs at all three localities, indicating that they are more faunivorous than the dwarf lemurs. Slightly elevated nitrogen values for dwarf lemurs in the fragmented forest may reflect increased consumption of arthropods at this locality. Alternatively, higher nitrogen isotope values for fragment individuals may indicate chronic nutritional stress. Future research will investigate these alternative options in depth.

Funding for this project was provided by the J. William Fulbright Program, Margot Marsh Biodiversity Foundation/CI Primate Action Fund, NSF BCS DDIG No. 0333078, Primate Conservation Incorporated, the Rufford Foundation, and a Saint Louis Zoo Field Research for Conservation Grant.